Liquid crystal display elements are advantageous in terms of their thin thickness, lightweight, and consumption of less power compared with other display elements. For that reason, liquid crystal display elements are widely used for image display devices such as TVs and monitors, OA (Office Automation) apparatuses such as word processors and personal computers, and image display devices mounted to video cameras, digital cameras, and information terminals such as mobile phones.
Conventional examples of liquid crystal display methods of liquid crystal display elements include: a twisted nematic (TN) mode using nematic liquid crystals; a display mode using ferroelectric liquid crystals (FLC) or anti-ferroelectric liquid crystals (AFLC); and a polymer dispersed liquid crystal display mode.
Among them, for example, a liquid crystal display element with a TN mode has come into practical use. However, the liquid crystal display element with a TN mode has disadvantages such as a slow response and a narrow viewing angle. These disadvantages prevent the liquid crystal display element to be superior to a CRT (cathode ray tube).
Although a display mode using FLC or AFLC has advantages such as a high-speed response and a wide viewing angle, the display mode using FLC or AFLC is greatly poor in terms of its anti-shock property and temperature property etc. Therefore, the display mode using FLC or AFLC has not come into practical use.
Further, the polymer dispersed liquid crystal display mode using light scattering does not require a polarizer and is capable of providing very bright display. However, the polymer dispersed liquid crystal display mode is unable to control a viewing angle by use of a phase plate, and has a problem in terms of response property. Accordingly, the polymer dispersed liquid crystal display mode has little superiority over the TN mode.
In all the foregoing display methods, liquid crystal molecules are aligned in a certain direction, and thus a displayed image looks differently depending on an angle between a line of vision and the liquid crystal molecules. On this account, all these display methods have viewing angle limits. Moreover, all the display methods utilize rotation of the liquid crystal molecules, the rotation caused by application of an electric field on the liquid crystal molecules. Because the liquid crystal molecules are rotated in alignment all together, responses take time in all the display methods. Liquid crystal display elements using the FLC or the AFLC are advantageous in terms of a response speed and a viewing angle, but have a problem in that their alignment can be irreversibly destroyed by an external force.
In contrast to those display methods in which rotation of liquid crystal molecules by the application of the electric field is utilized, there is proposed a display method in which the secondary electro-optical effect is utilized.
The electro-optical effect is a phenomenon in which a refractive index of a material is changed in response to an external electric field. There are two types of electro-optical effect: one is an effect proportional to the electric field and the other is proportional to square of the electric field. The former is called the Pockels effect: the latter is called the Kerr effect. The Kerr effect that is the secondary electro-optical effect was adopted early on in high-speed optical shutters, and has been practically used in special measurement instruments.
The Kerr effect was discovered by J. Kerr in 1875. So far, organic liquid such as nitrobenzene, carbon disulfide, and the like, are known as material showing the Kerr effect. These materials are used, for example, in the aforementioned optical shutters, optical modulation elements, and optical polarization elements. Further, these materials are used, e.g. for measuring strength of high electric fields for power cables and the like.
Later on, it was found that liquid crystal materials have a large Kerr constant at isotropic phase near nematic phase-isotropic phase change temperature (Tni). Researches have been conducted to utilize the large Kerr constant of the liquid crystal materials for use in light modulation elements, light polarization elements, and further optical integrated circuits. It has been reported that some liquid crystal compounds have a Kerr constant more than 200 times higher than that of nitrobenzene.
Under such circumstances, studies for using the Kerr effect in display devices have begun. As compared with the Pockels effect proportional to an electric field, the Kerr effect is expected to work for a relatively low voltage driving since the Kerr effect is proportional to a square of the electric field. Further, the Kerr effect is expected to be applied to high-speed response display devices since the Kerr effect inherently exhibits responding property of several microseconds to several milliseconds.
A significant practical problem to be overcome for the utilization of the Kerr effect in display elements is that utilization of the Kerr effect requires a higher driving voltage compared with conventional liquid crystal display elements. Another problem is that conventionally known materials that show the Kerr effect has a very narrow temperature range in which the Kerr effect is shown.
In order to solve the former problem, Patent Document 1 for example mainly proposes a technique where the surface of a substrate is subjected to an aligning treatment beforehand in a display element that drives molecules having liquid crystallinity at an isotropic phase state, so as to attain a state in which the Kerr effect is easily shown.
In the display element disclosed in Patent Document 1, molecules having liquid crystallinity are provided between a pair of substrates. Further, pectinate electrodes are formed on the inner side of one substrate (the side facing the other substrate), and alignment films having been subjected to a rubbing treatment are formed on surfaces of the substrates. Further, polarizers are provided on the outer sides of the substrates so that absorption axes of the substrates cross each other at right angles. Further, alignment films formed on the surfaces of the electrodes are provided (subjected to a rubbing treatment) so that rubbing directions of the alignment films are reverse-parallel or parallel with each other, and cross the absorption axes of the polarizers at an angle of 45 degrees.
In the display element of Patent Document 1 having the above structure, when a voltage is applied across the pectinate electrodes and an electric field is generated in a direction parallel to the surface of the substrate, molecules having crystallinity are aligned so that polarization of the molecules is in a direction of an electric field and a long axis direction of the molecules is parallel to the rubbing direction. This allows the display element of Patent Document 1 to lower a driving voltage required for showing the Kerr effect.
However, although Patent Document 1 discloses lowering the driving voltage, Patent Document 1 does not disclose any method for enlarging the temperature range within which the Kerr effect is shown. Consequently, it is difficult to put Patent Document 1 into practical use.
On the other hand, Patent Document 2 discloses a technique where a liquid crystal material to which a chiral agent is added is mainly used as a liquid crystal material for an optical modulation element in order to enlarge the temperature range within which the Kerr effect is shown, and a blue phase shown between the temperature range of a cholesteric phase (chiral nematic phase) and the temperature range of an isotropic phase is stabilized using a polymer network.
As with the isotropic phase, the blue phase is optically transparent, and exhibits optical isotropy. Further, a material showing the blue phase has a wider temperature range than a substance showing the Kerr effect only in a pure isotropic phase by a temperature range of the blue phase. However, even when the temperature range within which the Kerr effect is shown is enlarged, it is merely several ° C. (several K, several Kelvin). This is inadequate for putting Patent Document 2 into practical use.
For that reason, in Patent Document 2, the blue phase is stabilized using the polymer network, so that the temperature range within which the Kerr effect is shown is enlarged to approximately 60K for example.
Conventional examples of a display mode using a cholesteric phase (chiral nematic phase) include an NCPT (Nematic-Cholesteric Phase Transition) mode disclosed in Non-patent Document 1 and a PSCT (Polymer Stabilized Cholesteric Texture) mode disclosed in Non-patent Document 2. These display modes are mainly applied to a reflective display such as an electronic paper and a dispersion-transmission display that does not require a polarizer. In these display modes, a phase change and bistability among a planar state, a focal conic (fingerprint) state, and a homeotropic alignment state are applied to display, and desired alignment states are switched by optimization of a driving voltage pulse so as to utilize memory property of the state.
Further, Patent Document 3 discloses a technique where an electric field in a direction parallel to a plane of a substrate is applied on a cholesteric liquid crystal layer and the pitch of the cholesteric phase is changed, so that a wavelength of selective reflection changes from a UV range to a visible light range and attains color display. That is, in the technique disclosed in Patent Document 3, pitch P0 (natural chiral pitch P0) that is a spontaneous twist at a time of applying no electric field is set to the UV range and the electric field in a direction parallel to a plane of a substrate is applied so as to obtain pitch P1 or P2 that is larger than P0. Pitch of a twist at a time of applying an electric field or the wavelength of selective reflection is set to R (red), G (green), and B (blue) for example so as to attain color display. Further, Patent Document 3 discloses that three such cells are laminated and voltages to be applied on individual cells are adjusted so as to attain full color display. Further, Patent Document 3 discloses a technique where a polymer network is formed with the wavelength of selective reflection being in the UV range at a time of applying no electric field, so that elastic energy at the time of applying an electric filed is made effectively large, thereby speeding up restitution to the initial state (response speed of a relaxing process) at a time when application of the electric field is made off.
Further, Patent Document 4 discloses a technique where, in the display element for applying an electric field in a direction parallel to a plane of a substrate on a cholesteric liquid crystal layer, rubbing directions of alignment films formed on surfaces of upper and lower substrates are matched with axis directions of polarizers. Further, Patent Document 4 discloses that, in addition to the cholesteric liquid crystal layer to be driven, there is provided a cholesteric liquid crystal cell for optical compensation, that has a twist direction reversal to that of the cholesteric liquid crystal layer.    Patent Document 1: Japanese Unexamined Patent Publication No. 249363/2001 (Tokukai 2001-249363, published on Sep. 14, 2001)    Patent Document 2: Japanese Unexamined Patent Publication No. 327966/2003 (Tokukai 2003-327966, published on Nov. 19, 2003)    Patent Document 3: Japanese Unexamined Patent Publication No. 142823/1999 (Tokukaihei 11-142823, published on May 28, 1999)    Patent Document 4: Japanese Unexamined Patent Publication No. 189222/2002 (Tokukai 2002-189222, published on Jul. 5, 2002)    Non-patent Document 1: edited by The Japanese Association of Liquid Crystal Scientists, “Cutting edge of liquid crystal display”, p. 200-219, Sigma publishing, first edition, first printing, Oct. 10, 1996.    Non-patent Document 2: edited by Gregory Philip Crawford and Slobodan Zumer, “Liquid Crystals in Complex Geometries Formed by polymer and porous networks”, p. 103-142, Taylor & Francis, 1996    Non-patent Document 3: Takashi Kato and two others, “Fast and High-Contrast Electro-optical Switching of Liquid-Crystalline Physical Gels: Formation of Oriented Microphase-Separated Structures”, Adv. Funct. Mater., April, 2003, vol. 13. No. 4, p 313-317
However, as the display element disclosed in Patent Document 1 uses a pure isotropic phase of a liquid crystal material, it is impossible in principle to enlarge the temperature range for driving.
Further, as the technique disclosed in Patent Document 2 uses a blue phase that has not been put into practical use in a display device, the technique is unknown in terms of specific factors such as quality and reliability, and accordingly the technique is still suspicious in terms of durability against application of an electric field and reliability. That is, as the blue phase is an unstable phase in essence, even when the blue phase is stabilized using a polymer network, the blue phase is weaker than, for example, a nematic phase used in a conventional liquid crystal display device in terms of repeated switching on/off of an electric field and application of a high electric field. Therefore, it is expected that the blue phase be destroyed at an area near an electrode where the electric field is relatively strong.
Further, each of Patent Documents 3 and 4 discloses a display element that is driven by applying an electric field in a direction parallel to a plane of a substrate on a chiral nematic liquid crystal phase (cholesteric liquid crystal phase) having been put into practical use in a conventional liquid crystal display device. However, the display elements of Patent Documents 3 and 4 have, in essence, a slower response speed, a narrower viewing angle, and lower contrast than the display elements of Patent Documents 1 and 2 that utilize the Kerr effect to perform display. That is, the techniques disclosed in Patent Documents 3 and 4 cannot attain high-speed response property, wide viewing angle property, and high contrast property that are essential properties of the display device utilizing the Kerr effect.
The example embodiments presented herein were made in view of the foregoing problems. A feature of the present embodiments is to provide a display element and a display device that has high-speed response property, wide viewing angle property, and high contrast property, that has a wide temperature range for driving, and that is excellent in durability and reliability.
In order to solve the foregoing problems, the display element of an example embodiment is a display element, comprising: a pair of substrates, at least one of which is transparent; and a liquid crystal layer provided between the substrates, the liquid crystal layer being made of a medium having spontaneous twist pitch that is less than a wavelength of visible light, and the liquid crystal layer exhibiting optical anisotropy in a direction substantially parallel to a plane of each substrate (specifically, the direction forms an angle within a range of ±10 degrees (inclination) with respect to a plane of each substrate, and the direction is preferably parallel to a plane of each substrate) in response to application of an external field on the liquid crystal layer. The external field may be anything as long as it can change optical anisotropy of the medium. Examples of the external field include an electric field, a magnetic field, and light.
With the arrangement, the liquid crystal layer has spontaneous twist pitch less than a wavelength of visible light, and therefore exhibits optical isotropy in a direction substantially parallel to a plane of each substrate. When an external field is applied on the liquid crystal layer, alignment of liquid crystal molecules that constitute a medium changes and the liquid crystal layer exhibits optical anisotropy in a direction substantially parallel to a plane of each substrate. Here, a change in the degree of optical anisotropy means a change in the shape of refractive index ellipsoid. That is, the display element of the present embodiment realizes different display states by using a change in the shape of refractive index ellipsoid seen from a direction normal to a plane of each substrate at a time of applying no external field and at a time of applying an external field.
In contrast, a conventional liquid crystal display element applies an electric field on a medium so as to perform display. Refractive index ellipsoid maintains substantially the same elliptic shape at a time of applying an electric field and at a time of applying no electric field, and a long axis direction of the refractive index ellipsoid changes. That is, in the conventional liquid crystal display element, liquid crystal molecules are aligned in a uniform direction, and application of an electric field changes an alignment direction of the liquid crystal molecules (typically, rotates the liquid crystal molecules by 90 degrees) so as to realize different display states at a time of applying no electric field and at a time of applying an electric field. Therefore, the display element of the present embodiment has a principle for display that is greatly different from the conventional liquid crystal display element.
As described above, in the conventional liquid crystal display element, the amount of a change in an alignment direction of liquid crystal molecules is large, and as a result viscosity specific to a liquid crystal greatly affects response speed. In contrast, with the aforementioned arrangement, display is performed using a change in the degree of optical anisotropy of the liquid crystal layer. Therefore, with the aforementioned arrangement, viscosity specific to a liquid crystal does not greatly affect response speed unlike in the conventional liquid crystal display element. Accordingly, it is possible to realize higher response speed than the conventional liquid crystal display element. That is, the display element of the present embodiment switches between optical isotropy and optical anisotropy in a direction substantially parallel to a plane of each substrate in response to switching between application of an external field and application of no external field. Consequently, the display element of the present embodiment realizes response speed that is substantially as high as response speed of a conventional display element that performs display using the Kerr effect (response speed that is substantially a sub micro second).
Further, with the aforementioned arrangement, the display element of the present embodiment exhibits optical isotropy in a direction substantially parallel to a plane of each substrate at a time of applying no external field, and the display element of the present embodiment exhibits optical anisotropy in a direction substantially parallel to a plane of each substrate in response to application of an external field. Consequently, the display element of the present embodiment has wider viewing angle property than a conventional liquid crystal display element in which liquid crystal molecules are aligned in a uniform direction and alignment of the liquid crystal molecules is changed in response to application of an electric field so as to perform display.
Further, with the aforementioned arrangement, spontaneous twist pitch is less than a wavelength of visible light and as a result transmittance at a time of applying no external field is very high, which realizes high contrast.
Further, in a conventional display element that performs display using the Kerr effect, a temperature range at which the display element can be driven in response to a practical driving voltage is several K or so. Patent Document 2 discloses that stabilization of a blue phase by using a polymer network allows enlargement of a temperature range at which the Kerr effect is exhibited to approximately 60K for example. However, as the blue phase is an unstable phase in essence, even when the blue phase is stabilized using a polymer, the blue phase is weak in terms of repeated switching on/off of an electric field and application of a high electric field. Therefore, it is expected that the blue phase be destroyed at an area near an electrode where the electric field is comparatively strong.
In contrast, with the aforementioned arrangement, a medium for the liquid crystal layer is only required to include spontaneous twist pitch that is less than a wavelength of visible light. An example of the medium is a liquid crystal material that exhibits a cholesteric phase. The liquid crystal material that exhibits a cholesteric phase has been put in practical use in a conventional liquid crystal display element, and it is known that the liquid crystal material that exhibits a cholesteric phase maintains a stable phase structure within a wide temperature range (however, the technique for performing display by using spontaneous twist pitch less than a wavelength of visible light has not been disclosed). Accordingly, with the aforementioned arrangement, it is possible to realize a display element that has a wide temperature range for driving and that has excellent durability and reliability.
Further, in order to solve the foregoing problems, the display device of the present embodiment includes the display element of the present embodiment.
Accordingly, with the aforementioned arrangement, it is possible to realize a display device that has high-speed response property, wide viewing angle property, and high contrast property, that has a wide temperature range for driving, and that is excellent in durability and reliability. The display element of the present embodiment has high-speed response property as described above. Accordingly, by use of this property, the display element is applicable to a display device of a field sequential color mode of performing time sharing driving in which colors of a light source such as a backlight are switched at high speed in one unit field so as to perform display.